Final answer:
The Bolsheviks ended Russia's involvement in WWI by signing the Treaty of Brest-Litovsk with Germany in 1918, which resulted in significant territorial losses for Russia but allowed the focus to shift to the subsequent civil war.
Step-by-step explanation:
The Bolsheviks, led by Vladimir Lenin, brought Russia out of World War I by negotiating the Treaty of Brest-Litovsk with Germany in March 1918. This treaty formalized a ceasefire and ended the hostilities between Germany and Russia. The Bolsheviks agreed to cede vast territories, including Finland, Estonia, Latvia, Lithuania, Poland, Belarus, and Ukraine, which accounted for a substantial part of the Russian Empire's population and industrial base.
Although this resulted in significant territorial losses, it allowed the Bolsheviks to focus on internal issues and the civil war that ensued within Russia. The Red Army, representing the Bolsheviks, fought against various groups collectively known as the White Army and ultimately prevailed, leading to the establishment of the Soviet Union in the early 1920s.